The present invention relates to an image enhancer for detecting and identifying objects in turbid media. More specifically, but without limitation, the present invention relates to a laser system that can detect and identify objects in turbid media.
Laser systems have been and are continuing to be developed to detect and identify objects in turbid media (examples of turbid media include, but without limitation, seawater, clouds, and tissue). Operating a laser imaging system in such an environment is challenging due to the fact that light is both absorbed and scattered. Although the optical wavelength is typically selected to minimize absorption, the scattering experienced by an optical signal can severely degrade image quality. In highly turbid media, there may be plenty of light scattered back from the object of interest, but it is buried in the signal returning from the surrounding environment. A method for separating the unscattered (or minimally scattered) image bearing photons from the scattered, background light can be used to improve object detection and identification.
Several techniques have been developed for reducing the detrimental effects of scattered light. These approaches can be categorized according to the type of laser source and receiver combination and the scanning method used to create the image. All these types of systems are capable of creating an image, whether it is a synchronously scanned narrow beam and a narrow receiver field of view (narrow-narrow) or a flood-illuminated scene with a multiple pixel receiver (wide-narrow). The decision as to which configuration provides the best performance depends directly on the task at hand (i.e., above-water or below-water operation, size and depth of underwater object, water optical properties, etc.).
One type of known system is the Laser Line Scan (LLS) system. The Laser Line Scan (LLS) system includes a well-collimated continuous wave source and a narrow field of view receiver that are synchronously scanned over the object of interest. The bistatic configuration limits the common volume created by the source and receiver field of view overlap and reduces the contribution from scattered light. However, since the system uses a continuous wave source, no inherent time (depth) information is present in the detected signal, and post-processing using triangulation methods must be used to obtain object range information.
Pulsed laser sources are also used in several underwater laser-imaging systems to temporally discriminate against scattered light and to provide object range information. In the operation of a typical range-gated imaging system, a short (10-20 nsec) pulse is transmitted to a distant object, and the receiver is timed to open only when the reflected light returns from the object. A typical configuration is broad-beam illumination of the scene and a gated intensified camera receiver, although systems using photomultiplier tube receivers in both single and multiple pixel configurations have also been utilized. The Streak Tube Imaging Lidar (STIL) uses a pulsed laser transmitter in a ‘scannerless’ configuration. Instead of scanning the laser beam, a fan of light is used to illuminate a volume of water. The streak tube receiver can measure both the amplitude and range (time) of the collected slit of light, and a three-dimensional image is created when the system is operated from a moving platform. While the range-gated and STIL approaches are effective in minimizing background light, the sensitivity is ultimately limited by small-angle forward scattered light that induces image blurring.
Another type of underwater imager encompasses those that use temporal modulation of the transmitted light and subsequent synchronous detection of the modulation envelope at the receiver. The Underwater Scannerless Range Imager (USRI) uses a radio frequency modulation source that is coupled to both the timing of the laser transmitter and the gain of the image intensified CCD receiver. Object range information is obtained by measuring the phase difference between the transmitted and reflected signals simultaneously for each pixel of the receiver. However, multiple frames are required using different modulation schemes in order to extract the range information and to differentiate changes due to range variations from those due to intensity variations in the scene. Previous configurations used continuous wave sources, but a recent configuration implements a pulsed source and a range-gated receiver to minimize the volumetric backscatter signal.
Researchers at the Naval Air Systems Command (NAVAIR) are also developing a system that uses temporal modulation of the transmitted optical signal. However, in this approach, the optical receiver consists of a photodetector with sufficient bandwidth to recover the modulation envelope encoded on the optical signal. The resulting radio frequency signal is then processed using traditional radar signal processing techniques. This approach reduces the contribution by volumetric backscatter by using a modulation frequency that becomes strongly decorrelated with respect to the transmitted signal due to multiple scattering. A gain in image contrast is achieved when the modulation envelope emanating from an underwater object remains coherent relative to the original modulation signal. The phase information encoded on the detected modulation signal is processed to obtain object range information.
In previous embodiments of this modulated imaging system approach, a spectrum analyzer was used to produce the signal and the system produced a so-called “magnitude” image that was shown to have a nonlinear dependence on the object albedo (the term albedo may be defined, but without limitation, as ratio of the amount of electromagnetic energy reflected by a surface to the amount of energy incident upon it or the fraction of radiation striking a surface that is reflected by that surface). This non-linearity created a variety of sometimes unexpected features, such as contrast inversion, appearance of false elements in the image patterns, and the “emphasizing of the outlines” in object albedo patterns. In this system configuration, the irradiance distribution in the image plane is not proportional to the actual reflectivity of the object, the image is distorted, and its identification is difficult if impossible. This non-linearity is the main drawback of the conventional “magnitude” signal registration of previous system configurations.
For the foregoing reasons, there is a need for an image enhancer for detecting and identifying objects in turbid media.